This application is based on application No. JP 2000-274814 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an improved smooth impact drive mechanism (SIDM) that utilizes an electromechanical conversion element such as a piezoelectric element, and more particularly, to a smooth impact drive mechanism having improved stability. It further relates to an improved smooth impact drive mechanism that is capable of stable driving regardless of changes in the ambient environment such as changes in temperature.
2. Description of the Related Art
As a drive mechanism that utilizes an electromechanical conversion element such as a piezoelectric element that changes in length (i.e., extends and contracts) when the element is impressed with a voltage, a device such as that shown in the perspective view of FIG. 1(a), which shows the device in a disassembled fashion, and in the perspective view of FIG. 1(b), which shows the device in an assembled fashion, is known, for example.
This drive mechanism is capable of moving the moving unit 10 relative to the support platform 1, and may be used as the drive mechanism for the lens of a camera, for example. In other words, if the moving unit 10 is connected to the lens holder, the lens held in the lens holder may be moved together with the moving unit 10.
The piezoelectric element 4 comprises a number of piezoelectric plates stacked together. One end 4a of its length is fixed to the support platform 1, while the other end 4b is fixed to one end 5a of the rod (vibration member) 5. The rod is movably supported by the support members 2 and 3 that are integrally formed on the support platform 1.
The moving unit 10 incorporates the rod 5 using the main body 11 and cap 12 thereof, and by applying a downward force to the main body 11 and cap 12 using the pressure spring 13, the moving unit 10 becomes frictionally engaged around the rod 5.
To the piezoelectric element 4 is connected a voltage control circuit (drive circuit) not shown in the drawings. When a pulse voltage, which may be expressed as a sawtooth waveform as shown in FIG. 2, is continuously impressed to the piezoelectric element 4, the piezoelectric element 4 vibrates, i.e., extends and contracts, and the rod 5 vibrates along its length based on the movements of the piezoelectric element 4. Specifically, the piezoelectric element 4 extends relatively slowly at the gentle rising edge 101 of the first waveform 100, so that the rod 5 slowly moves in the direction of the arrow A in FIG. 1(b). When the voltage is suddenly dropped (as represented by the waveform portion indicated by the falling edge 102), the piezoelectric element 4 rapidly contracts and returns to its original length, whereupon the rod 5 rapidly moves in the direction of the arrow B.
Where the voltage is continuously applied such that identical waves 100, 100xe2x80x2, . . . are repeated, the rod 5 continues to vibrate while it repeats the slow movement in the direction A and the rapid movement in the direction B. In other words, the rod 5 vibrates while drawing a vibration waveform with gentle and steep portions. Here the spring force of the pressure spring 13 of the moving unit 10 (i.e., the frictional engagement of the moving unit 10 with the rod 5) is adjusted such that the moving unit 10 moves together with the rod 5 when the rod 5 moves slowly, and remains stationary (or moves over a smaller distance than the rod 5) due to inertia when the rod 5 moves rapidly. Therefore, while the rod 5 vibrates, the moving unit 10 moves in the direction A relative to the support platform 1.
Where the moving unit 10 is to be moved in the direction of the arrow B in FIG. 1(b), the pulse waveform of the voltage impressed to the piezoelectric element should be replaced with the one that is shown in FIG. 2 and that includes sharp rising edges and gentle falling edges. The principle of the movement of the moving unit 10 is the same as in the case explained above.
Incidentally, in a drive mechanism that utilizes an electromechanical conversion element, even if the drive waveform is unchanged, the moving speed of the moving unit may change based on the temperature of the electromechanical conversion element. FIG. 3 explains this nature, and shows that the moving speed of the moving unit increases as the temperature rises. In other words, because when driven continuously the temperature of the electromechanical conversion element increases as the time passes, its drive characteristics change over time.
The cause of the characteristic change shown in FIG. 3 may be fluctuation in the capacity of the electromechanical conversion element that occurs with a change in temperature. As shown in FIG. 4, the capacity declines as the temperature falls. Because the generated force of the electromechanical conversion element is proportional to the product of the capacity and size of the element, as the capacity declines, the generated force also declines, and as a result, the moving speed of the moving unit is reduced. xe2x80x98Capacityxe2x80x99 is the potential energy of the electromechanical conversion element, and is proportional to the dielectric constant of the element. Because the dielectric constant declines as the temperature declines, the capacity also declines as a result.
The present invention was created in view of the above circumstances, and an object thereof is to provide an improved smooth impact drive mechanism that utilizes an electromechanical conversion element, and more particularly, to provide improved stability. Another object of the present invention is to make improvement such that stable driving may be ensured regardless of changes in the ambient environment such as the temperature, i.e., to reduce fluctuation in the drive characteristics over time.
In order to attain these and other objects, the smooth impact drive mechanism according to one aspect of the present invention has a support platform, an electromechanical conversion element that is fixed to the support platform via one end thereof, a vibration member that is fixed to the other end of the electromechanical conversion element, a moving unit that is engaged with the vibration member using a prescribed frictional force, and a controller that controls the electromechanical conversion element and causes the moving unit to move relative to the support platform by causing the electromechanical conversion element to vibrate at different speeds depending on the direction of the vibration member, i.e., in the direction of extension or the direction of contraction, wherein the controller preheats the electromechanical conversion element before performing driving to move the moving unit. As one method of preheating, the controller impresses a preheating voltage to the electromechanical conversion element. The voltage impressed for the purpose of preheating has a drive waveform that does not as a practical matter move the moving unit. One specific example of such a drive waveform includes rising edges and falling edges that have the same angle of slant.
As described above, in general the characteristics of electromechanical conversion elements such as piezoelectric elements change in accordance with the temperature, such that the speed of the moving unit declines as the temperature declines. In the smooth impact drive mechanism of the present invention having the above construction, the electromechanical conversion element is preheated before the moving unit is actually driven, and therefore fluctuation in the speed of the moving unit may be reduced as a result, enabling the realization of stable drive characteristics with little speed fluctuation due to temperature change.
The smooth impact drive mechanism according to another aspect of the present invention has a temperature sensor that measures the temperature of the electromechanical conversion element, and the controller changes the drive waveform by which the electromechanical conversion element is driven in accordance with the output from the temperature sensor. As a specific example, the controller impresses the electromechanical conversion element with a voltage to preheat it until the output from the temperature sensor reaches a prescribed value. Alternatively, the controller impresses a preheating voltage for a prescribed period of time depending on the output from the temperature sensor at the time of startup of the device that incorporates the drive mechanism of the present invention. As another specific example, where the output from the temperature sensor is lower than the prescribed value, the controller increases the amplitude of the drive voltage or the drive frequency, and where the output from the temperature sensor equals or is higher than the prescribed value, the controller reduces the amplitude of the drive voltage or the drive frequency.
The smooth impact drive mechanism according to yet another aspect of the present invention has a speed sensor that detects the moving speed of the moving unit, and the controller changes the drive waveform by which the electromechanical conversion element is driven in accordance with the output from the speed sensor. As a specific example, where the moving speed of the moving unit, which is indicated by the output from the speed sensor, is lower than the prescribed value, the controller increases the amplitude of the drive voltage or the drive frequency, and where the moving speed equals or is higher than the prescribed value, the controller reduces the amplitude of the drive voltage or the drive frequency.
Using the above construction, the smooth impact drive mechanism can drive the moving unit at a stable speed despite such disturbances as fluctuations in temperature, etc.